1. Field of the Invention
This invention relates to an optical disk recording and reproducing apparatus which projects a light beam onto a rotating optical disk and records and reproduces the information, and more particularly to the tracking servo technique for causing the light beam to trace a track on the optical disk accurately.
2. Description of the Related Art
In an optical disk recording and reproducing apparatus, for the light beam to trace accurately a track where pits representing information are arranged in a row, a tracking servo is indispensable which generates a tracking signal and performs position control of the pickup across the radius of the disk on the basis of the tracking signal.
In a read-only optical disk, such as a DVD-ROM, pits representing information are recorded in the form of holes by press working. In a recordable optical disk, such as a DVD-RAM, a groove is formed in a spiral in such a manner that a groove as a concave portion and a land as a convex portion form a track. Projecting a beam onto the track causes pits representing information to be recorded in the form of phase changes at the disk surface. The beam reflected from the optical disk is received by an optical sensor composed of a plurality of light sensing cells. The light sensing cells each generate optical sense signals. Using these optical sense signals, a tracking error signal is generated.
A method of generating a tracking error signal includes a Differential Phase Detection (hereinafter, referred to as DPD) scheme and a Push Pull (hereinafter, PP) scheme. In the DPD scheme, a tracking error signal is generated on the basis of the phase difference between the optical sense signals outputted from the optical sensor. In the PP scheme, a tracking error signal is generated on the basis of the level difference between the optical sense signals outputted from the optical sensor.
The above-described conventional techniques, however, have the following drawbacks.
In the DPD scheme, a tracking error signal is generated using the diffraction of light by pits. Therefore, since no pit is formed in an unrecorded disk, it is impossible to generate a tracking error signal. Use of the phase difference permits a seriously erroneous tracking error signal to be generated, if there is a flaw at the disk surface, which leads to a faulty operation, such as track skipping.
In the PP scheme, the intensity of the reflected light in a recorded track differs from that in an unrecorded track, which results in the difference between tracking error signals. This makes the tracking servo unstable.